Restrictors, often called Thumpers by Resistance members, are tall, monolithic structures used by the Combine to deter Antlions. Using a heavy piston, they send strong vibrations into the ground which seem to frighten and ward off Antlions. They are used prolifically along coastal areas near to Nova Prospekt.

Restrictors are highly effective at deterring Antlions. If the device is active, the heavy central piston, consisting of three metal slabs, lifts up above the ground, before dropping back down to the earth, and delivering a powerful vibration. Whenever the piston hits the ground, Antlions will jump back and stay out of the effective radius of it.

Thumpers are deployed by the Combine. The Overwatch uses them extensively in Antlion-infested locations, placing them in strategic positions to prevent them over-running important areas. They are often utilized by Resistance members for protection in dangerous areas.

The most common restrictor type, found along the Coast near Combine-controlled areas as well as in some Resistance bases, is relatively small, but still effective. The ones seen around Nova Prospekt are larger and more massive, with their activation switches located at the top of a ladder.

Through the course of the chapter Highway 17, several restrictors can be seen and used along the coast to keep the Antlions at bay. Some, like the one at New Little Odessa, are used to keep Antlions out of more populated or mission-critical areas.

While crossing the Antlion-infested beach in the chapter Sandtraps, one restrictor can be found, but it is not operational. In order to get it working, Gordon is required to find and activate a generator trailer some distance away. It is notable that this restrictor is among the few items that are powered indirectly; the other being a force field generator, powered by an APC, that can be found in the Petroleum Station.

Nova Prospekt is surrounded by a network of restrictors to keep the hordes at bay. When Gordon Freeman attacks Nova Prospekt with the Antlions in his control, he is required to disable a number of the devices to allow the Antlions to assist him.

In Episode Two, a Restrictor is first encountered in one of the overrun Resistance outposts inside the Victory Mine. In the chapter Freeman Pontifex, after emerging from the Victory Mine, Gordon finds himself in a large open area. Three switch-less Restrictors can be seen in the valley, and become useful when Antlions crawl out of the ground, and two Antlion Guards rush out from the mines and begin attacking. Thumpers, surrounded by makeshift metal barricades, protect the player from both creatures and objects flung by the guards.

In the early stages of Half-Life 2's development, the thumper was to have a portable version. It can be found and used in the playable Half-Life 2 leak, where a yellow fire hydrant is used as a placeholder model.[2] It was simply named "Portable Thumper".[3] The player was to carry only one at a time and put it on the ground to achieve the same effects as the larger, stationary version. A second model, not appearing in-game and named "heavy", can be found in the game files.[2] In early Wasteland maps, early brush thumpers can be found.[4] They also appear as models in the playable Half-Life 2 leak files, the hammer being a separate model.[2]